The pilot reported that the helicopter was cruising at 2000 feet when, without warning, there was a complete loss of engine power. During the subsequent forced landing, the tail boom struck the ground, causing the tail rotor gear box to separate from the aircraft. Investigation revealed that the engine accessory gear train had failed, causing both magnetos to cease operation. The gear train failed due to lack of torque followed by the fatigue failure of six screws that attach the accessory drive gear to the starter gear. The cause of the loss of torque was not established.